Orthognathic surgery is used to treat severe congenital/ developmental craniofacial anomalies and dentofacial malocclusion. It is estimated that over 2.5% of the US population has severe dentofacial deformities requiring orthognathic surgery. Left uncorrected, these malocclusions can lead to psychological distress, impaired masticatory, speech and respiratory functions affecting the overall quality of life. Surgical techniques have improved over the past 50 years with rigid screw fixation decreasing post-operative complications and improving the success rate, yet surgical relapse still occurs. Approximately 20% of patients who had mandibular advancement surgery, with or without maxillary advancement surgery, experience moderate relapse 1-5 years after surgery. In addition, the success rate for more complex maxilla-mandibular surgery is reported to be as low as 60%. A principle component responsible for surgical relapse is due to improper positioning of the condyle and ramus during surgery. The displacement of the proximal segment of the mandible (ramus) produces a stretching of the muscles of mastication while patient is anesthesized. The stretch of the mastication muscles leads eventually to a relapse of the ramus to its original position. Therefore, intra-operative technique(s) or protocol that can assist surgeons in guiding the ramus/ condyle complex back into its biologically stable pre-operative position, should reduce surgical relapse and improve treatment outcomes. The goal of this project is to incorporate freehand ultrasound imaging technology with real-time 3-D image registration techniques to help Oral Maxillofacial Surgeons visualize the proximal segment mandible and guide it back into its stable pre-operative position. Our hypothesis is that a robust surgical navigation system with a real-time intra-operative visualization capability will improve the efficacy and safety of orthognathic surgery. The specific aims of phase I are (a) develop, prototype, and test algorithms for orthognathic surgery planning, navigation and visualization and (b) Integrate the algorithmic components into a complete Ultrasound-guided navigation software system and evaluate the system using a phantom. Following the Phase I feasibility demonstration, we will refine the system and conduct an extensive study in Phase II.